This invention relates to a magnetic field generator for use in an electromagnetic focusing type cathode ray tube, more particularly of the type utilizing a permanent magnet positioned on the outside of the tube to act as a flux generator for focusing an electron beam. More particularly, the invention relates to a device for compensating for the deterioration of the field intensity generated by the permanent magnet due to temperature rise.
Generally, an electromagnetic focusing type lens is more advantageous than an electrostatic lens in that its spherical and chromatic aberrations are small and its deterioration caused by space charge or the like is also small, so that it has an excellent resolving power.
A permanent magnet or an electromagnetic coil is used as a magnetic field generator for use in an electromagnetic focusing type cathode ray tube, but an excellent magnetic field generator can be formed by combining a low cost ferrite magnet and a magnetic member whose permeability varies greatly in accordance with temperature, as disclosed in Japanese Preliminary Publication of Patent No. Sho 54-55164.
FIG. 1 is a sectional view showing a basic construction of an electromagnetic focusing type cathode ray tube utilizing a permanent magnet acting as a focusing device of an electron beam, in which 1 represents a cathode ray tube, 2 a permanent magnet for generating magnetic flux and 3 lines of magnetic force generated by the permanent magnet 2. In this case, the permanent magnet 2 takes the form of a cylinder magnetized in the direction of the axis of the cathode ray tube 1 and mounted to surround a neck 1a containing an electron gun structure, not shown. The lines of magnetic force 3 are substantially parallel with the tube axis in the neck 1a for focusing an electron beam emitted by an electron gun structure. When the potential distribution in the neck 1a is determined, an optimum field intensity can be determined at once and when the field intensity is stronger or weaker than the optimum value, the size of an electron beam spot increases, thus degrading the resolution of a picture image.
FIG. 2 is an enlarged sectional view of a permanent magnet structure for producing magnetic field for focusing an electron beam which comprises an annular permanent magnet corresponding to the permanent magnet 2 shown in FIG. 1, first and second annular yoke plates 5a and 5b made of soft ferromagnetic material such as soft iron and secured to the opposite end surfaces of the annular permanent magnet 4 for rectifying the flux generated by the permanent magnet 4, a flux rectifying cylinder 6 closely surrounding the periphery of the permanent magnet 4 and made of a magnetic material, for example a Ni - Fe alloy, which varies its permeability according to temperature and is utilized to compensate for the temperature characteristic of the permanent magnet 4, and an adjustable cylindrical piece 7 made of such soft ferromagnetic material as soft iron and threaded onto the periphery of the first yoke plate 5a for finely adjusting the flux or field intensity between the first and second yoke plates 5a and 5b, the cylindrical piece 7 and the first yoke plate 5a constituting a field adjusting mechanism 8.
In the electron beam focusing magnet structure shown in FIG. 2, when the cylindrical adjusting piece 7 threaded on the periphery of the first yoke plate 5a is moved toward the second yoke plate 5b (in the direction of arrow B) the reluctance between the adjusting piece 7 and the second yoke plate 5b decreases so that the flux passing therebetween increases. Consequently, the flux along the tube axis decreases to weaken the intensity of the magnetic field acting upon the electron beam. On the other hand, when the adjusting piece 7 is moved toward the first yoke plate 5a (in the direction of A) reverse effect occurs. Thus, by moving the adjusting piece, it is possible to adjust the field intensity that focuses the electron beam.
With the magnetic field generator described above, however, as the temperature of the neck of the cathode ray tube rises to about 100.degree. C. from room temperature temperature of the entire magnet structure also increases. Accordingly, where a low cost ferrite magnet, for example barium ferrite or strontium ferrite magnet is used, the flux generated decreases at a rate of 0.2% per degree centigrade. As the temperature rises, the permeability of the flux rectifying cylinder 6 closely disposed about the periphery of the permanent magnet 4 decreases greatly so that the reluctance of the cylinder 6 increases to decrease the magnetic flux flowing therethrough. This compensates for the decrease in the flux along the tube axis, thus compensating for the temperature characteristic of the permanent magnet. The temperature characteristic compensation effect varies depending upon the material characteristic and the configuration of the flux rectifying cylinder 6.
With the magnetic flux generator described above, however, when the position of the adjusting piece 7 is varied for adjusting the field intensity, the reluctance between the first and second yoke plates 5a and 5b varies. At the same time, since the field intensity acting upon the flux rectifying cylinder 6 varies greatly the compensation effect of the flux rectifying cylinder 6 also varies greatly. In other words, even when the adjusting piece 7 is moved to an optimum position for adjusting the field intensity the temperature characteristic varies.